Due to the rapid expansion of GSM (Global System for Mobile Communications) networks, it is becoming more and more critical to effectively utilize the existing limited radio spectrum to support the increasing demand for mobile voice services. This has recently been achieved through the development of the so-called VAMOS (Voice services over Adaptive Multi-user channel on One Slot) feature within the 3GPP standardization framework, GSM has evolved from full rate (FR) where one user occupies one time slot, to half rate (HR) where two users are multiplexed on the same time slot. With VAMOS pairs of users can be multiplexed, or paired, onto two VAMOS subchannels of a single physical radio resource to support up to four users on a single timeslot, as illustrated in FIG. 1. Consequently, by implementing VAMOS it is possible to double the number of users that can be handled by each radio base station transceiver. This enhances the hardware efficiency and increases the spectral utilization of GSM while reducing the footprint of GSM radio base stations.
VAMOS doubles the uplink (UL) speech capacity by allowing a pair of VAMOS-allocated mobiles to share the same carrier frequency and time slot number. The mobiles utilize GMSK (Gaussian Minimum Shift Keying) modulation and the VAMOS solution relies on RBS multi-user detection algorithms to demodulate the two GMSK signals.
In downlink (DL) two VAMOS mobile are served by a single AQPSK (Adaptive Quadrature Phase Shift Keying) modulated carrier, see FIG. 2. The AQPSK modulated signal equals the superposition of two GMSK modulated carriers, separated by a 90 degrees phase shift. According to VAMOS terminology, each carrier is allocated to a VAMOS subchannel. In DL the VAMOS solution typically relies on the capability of each mobile station to suppress the synchronous co-channel interference caused by the paired mobile allocated in the paired VAMOS subchannel.
It has recently been discovered by several different stakeholders that some legacy GSM mobiles are not VAMOS capable in the downlink (DL). Since this problem does not occur with mobiles allocated to a non-VAMOS allocation (using GMSK modulation) it is the inherent co-channel interference present in a VAMOS allocation (using AQPSK modulation) that causes call drops see [1], [2], and [3]. At the same time, VAMOS is heavily dependent on the backwards compatibility with legacy mobiles to get market acceptance
Observations has been made that activation of discontinuous transmission (DTX) improves the performance. The explanation is that when one mobile becomes inactive in the DL (no speech activity), the BTS (Base Transceiver Station) will utilize GMSK modulation to transmit the bursts to the active paired mobile, even if the mobiles are in a VAMOS allocation. The conclusion is that it is enough for a legacy mobile allocated in VAMOS mode to occasionally receive GMSK modulated bursts to maintain a call. However, since DTX is a statistical property it is not a reliable solution to this problem. E.g. in situations with high level of background noise or in telephone conferences there could be long call durations without the connection entering DTX mode, i.e. an inactive state.
As the VAMOS solution is heavily dependent on the support of legacy mobiles this discovery has the potential to cause permanent damage to the implementation and reputation of VAMOS.
Consequently, there is a need to provide reliable solutions that prevent legacy mobiles e.g. user equipment from experiencing call drops when paired with a VAMOS capable mobile.